基于谐波电压突变的无功功率扰动孤岛检测法

由于无法辨析并网点(point of common coupling, PCC)电压跌落的本质原因，光伏电站低压穿越(low voltage ride through, LVRT)控制与现有的孤岛保护在同时满足动作条件时缺乏选择性，影响系统的安全稳定。为研究与光伏LVRT相互协调的孤岛检测方法，根据孤岛现象和电压暂态扰动现象发生时PCC谐波电压的差异，提出一种基于光伏并网点谐波电压突变的无功功率扰动孤岛检测方法。该方法在检测到PCC处的谐波电压突变后，经40 ms延时，对光伏逆变器输出的无功电流采用基于PCC频率变化量函数进行扰动，使PCC频率超出正常范围，实现孤岛检测功能。仿真结果表明：方法在保证与LVRT协调配合的前提下能在较短时间内完成对光伏电站孤岛状态的检测；此外在故障特征量较小的工况下方法依然可靠适用，消除了传统孤岛检测方法盲区的同时保证了孤岛检测的准确性。     

基于谐波特性的光伏孤岛与低电压穿越同步检测方法

针对光伏并网系统孤岛保护与低电压穿越之间的运行冲突问题,提出了一种基于谐波特性的光伏孤岛与低电压穿越同步检测方法。根据系统在并网、孤岛、电压扰动三种运行状态下谐波电压的不同,用傅里叶级数定量分析了频谱泄漏对谐波检测的影响,同时在考虑主电网背景谐波对谐波检测的影响后,推导了谐波电压阈值的整定公式。在此基础上,给出了同步检测实现方案及其算法流程,并在仿真软件平台上验证。从“协鑫边昭光伏电站带边昭变孤岛试验”的数据中提取的谐波特性曲线进一步验证了所提的谐波电压阈值可以识别孤岛现象。理论分析、仿真实验以及现场实验都证明了所提的谐波电压阈值可以作为区分孤岛现象和电压暂态扰动现象的依据,所提的基于谐波特性的同步检测方法可以有效协调光伏并网系统孤岛保护和低电压穿越两种功能。     

含PQ控制逆变型分布式电源的配电网故障分析方法

随着逆变型分布式电源(inverter interfaced distributed generation,IIDG)在配电网中的渗透率不断提高,且在新的故障穿越行为要求下,现有的故障分析方法已不再适用。为此,提出一种适应于最新并网规定的含PQ控制IIDG的配电网故障分析方法。该方法首先通过分析IIDG故障电流特性,提出计及控制特性的IIDG压控电流源等值模型;在此基础上,建立了故障下的含IIDG配电网节点电压方程,并针对不同IIDG间相互耦合以及公共连接点(point of common coupling,PCC)故障电压与IIDG故障电流之间存在非线性关系,提出相应的迭代修正求解方法。最后,通过算例仿真计算,验证了所提方法的可行性和有效性。     

含PQ控制逆变型分布式电源的配电网故障分析方法EI北大核心CSCD

随着逆变型分布式电源(inverter interfaced distributed generation,IIDG)在配电网中的渗透率不断提高,且在新的故障穿越行为要求下,现有的故障分析方法已不再适用。为此,提出一种适应于最新并网规定的含PQ控制IIDG的配电网故障分析方法。该方法首先通过分析IIDG故障电流特性,提出计及控制特性的IIDG压控电流源等值模型;在此基础上,建立了故障下的含IIDG配电网节点电压方程,并针对不同IIDG间相互耦合以及公共连接点(point of common coupling,PCC)故障电压与IIDG故障电流之间存在非线性关系,提出相应的迭代修正求解方法。最后,通过算例仿真计算,验证了所提方法的可行性和有效性。     

IIDG低压穿越模型及其在配网故障分析中的应用

在我国并网规程对分布式电源低压穿越功能的要求下,为了实现逆变型分布式电源IIDG(inverter-inter-faced distributed generator)的最优工作效率,提出了基于无功电流支撑的低电压控制策略。通过增加电压跌落门槛值实现输出有功电流的优化,并分析了采用该策略时逆变型分布式电源的故障特性。在此基础上,建立了含IIDG的配电网短路计算模型。DIGSILENT仿真表明该低电压穿越策略在电压跌落时可实现IIDG的并网运行和最大的有功输出。含IIDG的配电网短路计算模型考虑了低压穿越策略,为大规模分布式电源并网的继电保护方案提供参考。     

含逆变型分布式电源配电网的新型纵联保护方案

中小容量逆变型分布式电源(IIDG)分散接入使传统单源中压配电网变为多源网络,常规过电流保护难以适用,且基于电力电子接口的分布式电源外输出特性和故障暂态特性与同步发电机相比存在明显差异。为此,提出了适用于有功/无功解耦控制(PQ控制)IIDG的等值模型,该模型能够准确刻画出具备低电压穿越能力IIDG的电流输出特性和故障暂态特性。在此基础上,通过研究馈线公共联接点(PCC)正序电压计算值的分布特征,在PCC线路两侧无需增加额外设备的原则下,提出了适用于高渗透率IIDG分散灵活接入的配电网全线速动双端纵联保护方案,降低了多端保护对纵联通道的需求,提高了电网经济性。最后,通过PSCAD/EMTDC验证了所提等值模型的准确性以及保护方案的有效性。仿真结果表明,由该等值模型所获取的IIDG电流的最大相对计算误差≤5%,且所提保护方案不受IIDG出力、故障类型、故障位置等因素的影响。     

基于阻抗辨识的下垂控制并网逆变器孤岛检测方法

为了保障安全及并网电能质量,基于并网逆变器的分布式发电系统需要具备孤岛检测功能。传统孤岛检测方法不适用于下垂控制并网逆变器,而现有适用于下垂控制并网逆变器的孤岛检测方法需要改变下垂特性且存在检测盲区的问题。为了解决上述问题,提出一种基于阻抗辨识的下垂控制并网逆变器孤岛检测方法。该方法在公共耦合点注入电压扰动,通过电压、电流扰动信号辨识阻抗,进而依据阻抗变化判定是否出现孤岛效应。利用谐波扰动注入放大孤岛前后特征量差异,提高检测灵敏性。另外,通过递归离散傅里叶变换提取扰动信号,提高了阻抗辨识的准确度和时效性。实验结果验证了所提方法的有效性。     

基于功率指令切换的双级式构网型光伏故障穿越控制策略

在新型电力系统构建中,构网型光伏将发挥巨大作用。但受限于电力电子器件过流能力,目前针对构网型光伏的故障穿越控制策略还需进一步完善。综合考虑逆变器稳态和暂态过流抑制,提出了一种基于功率指令切换和虚拟阻抗的双级式构网型光伏发电故障穿越控制策略,该策略计及光伏直流侧动态响应,能够实现满足电流限幅约束的前提下向并网电压提供无功支撑,同时保持故障期间构网型光伏对系统惯性的支撑特性,提升大扰动下光伏换流器暂态功角稳定性。仿真结果表明,在电网发生严重电压跌落的情况下,所提控制策略可在保持逆变器直流侧电压稳定的同时实现故障穿越功能。     

新能源并网发电系统低电压穿越与孤岛同步检测的无功功率扰动算法

随着新能源并网发电系统的大量建设与并网,准确、快速地实现低电压穿越与孤岛检测是并网系统的两项重要功能。实际运行时,依据并网准则定义的低电压穿越与孤岛检测的操作时间,并网系统需要能够同步进行低电压穿越运行与孤岛检测,一旦检测到孤岛状态即执行停机操作。但是现有的低电压穿越与孤岛检测方法,在时间和结构上都难以进行简单的组合,无法同时实现低电压穿越与孤岛检测。提出一种由基于双二阶广义积分器的锁相环、功率扰动模块以及孤岛检测模块组成的无功功率扰动方法。该方法控制新能源并网发电系统输出无功电流,同时利用输出的无功功率进行功率扰动,可同时满足低电压穿越运行与孤岛快速检测的要求。最终,仿真结果验证了该方法的有效性与准确性。     

基于工频变化量的序分量距离保护

为了适应孤岛运行低电流值的故障特性,一般分布式电源孤岛运行时的保护方法与并网运行时的不同。但是随着分布式电源投入和切除配电网频率的增加,保护的整定值也必须频繁地在不同保护方法之间转换。所以提出了基于工频变化量的序分量距离保护,以短路故障时负序电压为例,对其电压分布进行分析,得出了负序和零序继电器的动作判据,并在Matlab中搭建负序和零序继电器的仿真模块。通过仿真分析了在并网和孤岛两种情况下,该保护方法在不同故障距离和过渡电阻情况下的动作情况,从而验证了该保护方法在并网和孤岛两种方式下均能够可靠动作。     

Disturbance detection in grid-connected distributed generation system using wavelet and S-transform

In this paper, wavelet transform and S-transform based approach is proposed for islanding detection and disturbance due to load rejection in distributed generation (DG) based hybrid system. The two types of distributed generation technology: inverter based and rotating machine based, that consists of photovoltaic, fuel cell and wind systems are considered in hybrid system configuration. The negative sequence voltage signal is analyzed through wavelet transform and S-transform for islanding detection of these resources from the grid. The above two approaches are also used in study of voltage profile at point of common coupling (PCC) with a non-linear load connected. The study for different scenarios in operation of DG system is presented in the form of time-frequency analysis. The energy content and standard deviation (STD) of S-transform contour and wavelet transform signal is also reported for both islanding detection and disturbance due to load rejection.     

基于自适应负序电压正反馈的孤岛检测方法

孤岛发生后,在负序电压正反馈环节作用下,并网点产生的负序电压持续增大,通过判断并网点负序电压幅值是否超过阈值进而判断孤岛是否发生。但是,过大的反馈系数会导致逆变器注入负序扰动过大,过小的反馈系数会导致孤岛检测失败。根据负序电压正反馈法的原理提出了基于自适应负序电压正反馈的孤岛检测方法,其具有足够孤岛检测能力的正反馈系数取决于逆变器实时输出功率的大小。改进后的方法在逆变器并网运行时能有效减小注入大电网的负序扰动分量,且孤岛发生后能够在规定时间2 s内实现孤岛检测。通过理论分析和仿真研究,验证了所提方法的可靠性。     

Two level islanding detection method for distributed generators in distribution networks

Use of distributed generators in distribution networks is proved highly reliable and economical. In utility practice at the present time, accidental islanding is an undesirable mode of operation as it can harm the personnel and other connected systems. Therefore, islanding must be detected and the islanded distributed generators must be disconnected from the rest of the system. This paper proposes a new islanding detection method which provides a reliable detection in two levels. At first level wavelet energy entropy is used as an indicator to suspect islanding and activate second level method. Second level method is an active frequency drift method with positive feedback and pulsating chopping fraction that vary the output voltage frequency of distributed generators to realize the anti islanding protections. The performance of the proposed method is simulated in MATLAB environment. The proposed algorithm provides fast islanding detection with least disturbance in inverter’s grid connected operation.     

带负载阻抗角反馈的主动频移孤岛检测技术

孤岛检测是分布式光伏并网发电系统稳定可靠运行的关键技术之一,而传统的主动式孤岛检测技术难以平衡检测准确性和电能质量之间的矛盾。因此,详细分析了负载特性对孤岛检测的影响,针对并/离网逆变器模型提出了一种带负载阻抗角反馈的主动频移孤岛检测技术,通过反馈实时负载阻抗角改变注入的电流频率扰动,提高了单台逆变器和多逆变器光伏并网发电系统孤岛检测的准确性,消除了检测盲区。同时,通过提出的孤岛检测方法进行分段式改进,降低了由非线性因素引起的并网电流谐波畸变率,改善了系统的电能质量。仿真和实验均验证了所提孤岛检测方法的有效性和可行性。     

基于周期交替电流扰动的孤岛检测方法

对于分布式电源并网系统,当电网断电时可以使用主动频率偏移法检测孤岛效应。但是,若在非检测区内电流扰动角与负载角相匹配,则主动频率偏移法将出现无法识别孤岛效应的问题。针对上述问题,文中提出了一种新颖的基于周期交替电流扰动的孤岛检测方法。在保证扰动电流与公共耦合点电压同步前提下,前一个周期扰动电流频率向上偏移,当前周期扰动电流频率向下偏移。根据公共耦合点电压频率是否周期高低交替变化来识别孤岛现象。经过仿真和实验验证,该方法不但可以准确识别孤岛效应,而且解决了主动频率偏移法存在的非检测区问题。     

Harmonics-Based Enhanced Passive Islanding Method for Grid-Connected System

Now a day, distributed sources are emerging widely due to their advantages owing to renewable-based power generation. In this paper, a novel passive islanding method for the inverter-based grid-connected system is presented. This method can be suitably applied along with the conventional passive methods like under/over voltage and under/over frequency islanding techniques. Total harmonic distortion (THD) at the point of common coupling (PCC) is used to discriminate the islanding condition, and to safely disconnect the grid. This method has an advantage that it is able to discriminate non-detection zone (NDZ) when the local load matches the inverter output power. The results using MATLAB simulations and laboratory prototype have been presented.     

An active islanding detection scheme for inverter-based DG with frequency dependent ZIP–Exponential static load model

In this paper an analytical design method is proposed for constant voltage controlled inverter based DG with frequency dependent ZIP–Exponential static load model to obtain proper inverter output control and successful islanding detection. The proposed technique is aimed to eliminate the dependency of DG operation point on anti-islanding performance which is unavoidable in case of traditional control strategy. The generalized formulae are derived for calculating the upper and lower limits of the voltage gain, and it is recommended to select the voltage gain within the designed range for effective islanding detection. The proposed approach is tested in MATLAB/Simulink environment and result shows that the proposed scheme is found to be highly effective in islanding detection considering complex network configurations with single and multiple DG systems. The proposed control scheme is able to detect islanding within 10 ms (half cycle) from the event inception, showing the fastness of the developed technique. It is also observed that the percentage of voltage and current THD of the proposed scheme is within permissible limits, indicate that the power quality of the inverter is not degraded by the proposed control strategy, and thus overcomes the drawback of existing active anti-islanding techniques.     

Active islanding detection method for the grid-connected photovoltaic generation system

This paper proposes an active islanding detection method incorporated into the control of the grid-connected inverter to protect the photovoltaic generation system from the islanding operation. The proposed active islanding detection method performs the grid-connected inverter as a virtual resistor with the operation frequency slightly higher or lower than the fundamental frequency of the utility voltage. The function of virtual resistor will not be actuated when the utility is nominal, and the grid-connected inverter can convert the DC power from the solar array to an AC power. When the strong utility is lost, the grid-connected inverter acts as a virtual resistor with the operation frequency slightly higher or lower than the fundamental frequency of the utility voltage. Thus, the frequency and the amplitude of the local load voltage will be away from their normal values under the islanding operation. Hence, the proposed active islanding detection method can immediately detect the islanding operation. A prototype is developed and tested to demonstrate the performance of the proposed active detection method. Both computer simulation and experimental results verify that the performance of the proposed active islanding detection method is expected.     

改进的主动频率偏移孤岛检测算法

针对被动孤岛检测存在检测盲区以及传统主动频率偏移(AFD)孤岛检测算法会带来有功电流波动的不足,提出一种改进的AFD孤岛检测算法。该算法通过控制无功电流的大小来实现电流频率偏移,从而实现孤岛状态下接入点电压频率的正反馈,进而检测出孤岛。为了减小无功电流对电力系统的污染,需将该AFD算法与传统的被动孤岛检测算法结合使用。仿真和实验结果表明,该孤岛检测算法不仅消除了被动孤岛检测算法在负载功率和逆变器功率匹配时存在的检测盲区,而且克服了传统AFD算法有功电流波动的缺点,提高了直流母线电压的稳定性。